This invention relates to improvements in electrostatic precipitators, and more particularly in single-stage electrostatic precipitators of the type in which gas-borne particles subjected to the high-voltage electric field effects of discharge filament electrodes become charged so as to be attracted to and held by adjacent collector plate electrodes polarized oppositely to the discharge filament electrodes. The invention is herein illustratively described by reference to its presently preferred embodiment; however, it will be recognized that certain modifications and changes therein with respect to details may be made without departing from the essential features involved.
Certain prior art disclosures of background interest relative to such electrostatic precipitators and to an understanding of how certain prior art precipitators have operated are as follows: U.S. Pat. Nos. 1,791,338 Wintermute (1931), 2,654,438 Wintermute (1953), 3,740,925 Gothard (1973), Publications: White, H. J. "Industrial Electrostatic Precipitation", Addison-Wesley, Reading, Massachusetts, 1963. Oglesby, S. and Nichols, C. B. "A Manual of Electrostatic Precipitator Technology", Southern Research Institute, Birmingham, Alabama, 1970.
In prior art precipitators most commonly used, pairs of opposing parallel flat collector plate electrodes are provided with elongated flow interrupter ribs projecting into the flow stream at interval locations along the mutually opposing faces of the electrodes. The individual ribs extend parallel to the discharge filament electrodes also spaced at intervals along the gas flow channels between the plates. The intended effect of these flow interrupter ribs is to retard the gas flow adjacent the electrode surfaces both immediately ahead of each rib and immediately behind it thereby to minimize gas flow scouring effects causing removal and reentrainment of the particles held by the plates and also so as to afford the associated newly incident particles greater dwell time in the vicinity of the collector plates to enhance their prospect of capture. The high voltage applied between the filament and plate electrodes establishes an electric field characterized by high intensity in the region immediately surrounding the discharge filament electrodes and relatively low but nonetheless substantial intensity in the region immediately adjacent the collector plate electrodes. Sparkover, which is a sudden electrical discharge between a filament and a plate electrode, determines the upper limit for the voltage applied between these electrodes. In operation, gas-borne particles entering the corona region of the discharge filament electrodes accumulate a static charge by gathering free ions released in the corona discharge. The upper limit of this charge accumulation, reached at an equilibrium condition in the electric field, increases as the strength of the electric field is increased. Each charged particle experiences an electrostatic attraction force toward the nearest collecting electrode and of a magnitude proportional to the particle charge and to the intensity of the electric field (i.e., the voltage gradient). However, in the central core flow region between adjacent plate electrodes where the velocity is high and the flow is highly turbulent, the electrostatic force is small in relation to the aerodynamic forces that tend to make the particle follow the motion of the gas. Thus, the aforementioned electrostatic attraction force has little effect on particle position until the particle is swept by the turbulence into a region of reduced flow closely adjacent the collector plate. In this region the migration velocity induced by the electric field force dominates, and the particle is captured by the collecting electrode and by residual attraction, held in an accumulating layer. Removal of the accumulated layer of particles is commonly accomplished by periodically striking the top edge of the plates with a hammer mechanism so that large chunks or sheets of the collected material fall into hoppers below the plates. This process is called rapping. During a collector plate rap not all of the dislodged particles fall into the hoppers. Some of them are accelerated out of the region of reduced flow adjacent the plates and are reentrained into the main flow.
The separated flow regions (i.e., regions of retarded gas flow) created by the flow interruption effect of these special ribs mounted at intervals along the collector electrode plates are intended to shield the collected particle layers from direct scouring by the main flow stream and to inhibit reentrainment of the particles into the main flow, particularly during rapping. In so doing, however, the ribs also unduly impede the main flow between electrodes and seriously lower the sparkover voltage threshold of the precipitator. Sparkover threshold determines the practical upper operating limit for the applied electrode voltage, and thus the upper limit for the electric field intensity in the inter-electrode space. High electric field intensity in the particle discharge region promotes a high charge for the particles, which is desirable since the electric force on the particle is proportional to the particle charge. It is also desirable that the field intensity be high as possible in the regions near the collector electrode plates where the electrostatic forces effect capture since these forces are proportional to the field intensity. Also it is advantageous for the field to be as uniform as possible in these regions. Non-uniformity in the electric field along the surfaces of the collecting electrode plates is undesirable for two reasons. First, severe variations, such as those occurring near a sharp projection on an electrode plate, promote sparkover. Second, reduction of intensity along an electrode plate from the peak value allowed by sparkover design limitations reduces the average migration velocity of particles in the capture region and thereby the capture efficiency of the system as a whole. In general, the desirable conditions in the capture region near the collector plate are minimum gas flow velocity, highly charged particles, and a uniformly high electric field intensity. Mechanical rigidity is also desirable for the collector electrode plates in order to maintain electrode plate shape and thereby the intended precise positional relationship between opposing electrodes. Also desirable is the ability of the plate to transmit vibratory motions from rapping at the top of the plate to all parts of the collector plate.
In the above-cited "A Manual of Electrostatic Precipitator Technology", at pages 231 and 232 discharge filament electrodes are disclosed in combination with electrode plates of zig-zag, offset or corrugated configuration. However, the configurations there disclosed retain the effect of abrupt prominences that promote sparkover to the discharge filament electrodes and non-uniform electric field intensity along the electrode plate and therefore possess most of the characteristics and problem limitations of the commonly used ribbed plate electrodes discussed above. Thus although such previously disclosed configurations may bear superficial resemblance to the present invention, the referenced disclosure differs significantly from it and fails to recognize and overcome those limitations and fails to provide an enabling disclosure of this invention.
A broad object of the present invention therefore is to provide an improved configuration for the electrodes of electrostatic precipitators of the described type, and more specifically an improved configuration for the collector plate electrodes that increases the particle capture efficiency thereof and the particle retention capacity of such devices without unduly impeding the main flow through the precipitator.
A related object hereof is to provide such an improved precipitator that can be made of more compact and simpler construction hence less costly when installed than the prior art devices of the ribbed plate and filament array type described above.
A further and more specific object of this invention is to provide a more effective precipitator collector electrode plate configuration that permits the applied voltage to be increased over that usable with prior art devices of corresponding collector plate spacing, yet without exceeding the sparkover voltage threshold. A related object is to simultaneously improve the particle capture electric field pattern and the gas flow pattern in such precipitators conducive to operating efficiency and physical design compactness while providing collector electrode plates of the requisite form retention rigidity and rapping vibration transmission properties. In the attainment of such objectives not only are more highly charged particles and more intense and more effective electric field patterns obtainable, but the gas-borne particles carried into the reach of the collector plate electrodes are caused by the gas flow stream to dwell or be decelerated for a longer time period conducive to completing their migration to the attractive collector plate surfaces. Furthermore, the invention concurrently provides improved captive particle retention characteristics in such precipitators, due not only to the higher operating voltage limits attainable therein, but also due to the improved gas flow effects of the collector plate electrode configurations that reduce the scouring tendencies of gas flow otherwise presenting problems in the dislodgement and reentrainment of captive particles.
As herein used, the term "filamentary" or "filament" applied to the discharge electrodes is intended to mean and include a wire, rod, bar, cable, strip, or generally any elongated configuration that is cross-sectionally narrow in any direction relative to its length. In other words, the discharge electrode member configuration being defined is understood to be of such restricted cross-sectional dimensions in relation to the expanse between successive similar members in the same array that electric flux lines passing between the respective members and the adjacent collector plate electrodes will converge on the members and at operating voltages produce corona discharge in the immediately surrounding vicinity. A round wire or similar elongated discharge filament electrode is the simplest and preferred form of such member, but it is to be understood that other configurations are also usable within the intended definition.
The terms "corrugation" and "corrugated," as herein used with relation to the configuration of the paired collector plate electrodes or to that of the mutually opposing surfaces of such electrodes, imply curvilinear surfaces without abrupt or sharp contours that can produce undue electric field concentrations. These terms also have reference to wavy or sinously shaped surfaces, that is, surfaces that have alternate crests and intervening valleys. Preferably they are shaped as sine waves; however, surface shapes deviating somewhat from a pure sine wave may also be used. The successive alternating crests and valleys defined by these individual collector plate electrode surfaces are mutually parallel and are aligned perpendicular to the direction of gas flow past such surfaces. Moreover, in the required configuration and spatial relationship between opposing corrugated collector electrode plate surfaces the crests of one oppose the crests of the other so as to form flow channels that alternately converge and diverge along the direction of flow, and such corrugation alignment is herein said to be of opposite phasing. The terms divergence-convergence region and convergence-divergence region, as used herein, respectively refer to a wide portion and a narrow portion of the channel between opposing corrugated collector plates.
The term "corrugation height" as herein used refers to the distance between a crest peak and a valley low point measured normal to the mean plane of a corrugated surface, and the term "corrugation wave length" refers to the distance between successive peaks.